1. Field of the Invention
The present invention relates generally to an electrophotographic apparatus; and more specifically, to an improved structural arrangement having a densitometer. Moreover, the densitometer arrangement achieves improved measuring of marking particle density on a substrate. Specifically, the densitometer is responsive to both changing environmental conditions and differences between individual machines.
2. Description of the Prior Art
It is known in the electrical graphic arts to use light sensors for measuring the density of a powderous substance or the like. One such sensor is a developability sensor, also known as a densitometer, used to monitor the "Developed toner Mass per unit of Area," referred to as DMA. Current developability sensors are optically based. The sensors are required to monitor the DMA of both black and colored toners. For example, in co-pending U.S. patent application Ser. No. 07/399,051, it describes a densitometer which measures the reduction in the specular component of the reflectivity of a portion of a surface having a liquid color material deposited thereon. Collimated light rays, in the visible spectrum, are projected onto the portion of the surface having the liquid thereon. The light rays reflected from the portion of the surface having the liquid deposited thereon are collected and directed onto a photodiode array. The photodiode array generates electrical signals proportional to the total flux and the diffuse component of the total flux of the reflected light rays. Circuitry compares the electrical signals and determines the difference therebetween to generate an electrical signal proportional to the specular component of the total flux of the reflected light rays.
Similarly, co-pending U.S. patent application Ser. No. 07/246,242, which is herein incorporated by reference in its entirety, describes an infrared densitometer which measures the reduction in the specular component of reflectivity as toner particles are progressively deposited on a moving photoconductive belt. Collimated light rays are projected onto the toner particles. The light rays reflected from at least the toner particles are collected and directed onto a photodiode array. The photodiode array generates electrical signals proportional to the total flux and the diffuse component of the total flux of the reflected light rays. Circuitry compares the electrical signals and determines the difference therebetween to generate an electrical signal proportional to the specular component of the total flux of the reflected light rays.
U.S. Pat. No. 4,950,905, which is herein incorporated by reference in its entirety, discloses a color toner density sensor. Where, light is reflected from a toner predominantly by either scattering or multiple reflections to produce a significant component of diffusely reflected light. Moreover, part of the sensor is arranged to detect only diffusely reflected light, and another part is arranged to detect both diffuse and specularly reflected light. In operation, the diffusely reflected light signals are used to identify increasing levels of diffusely reflected light which in turn indicates an increased density of toner coverage per unit of area.
U.S. Pat. No. 4,801,980, discloses a toner density control apparatus having a correction process. The object of the invention is to prevent a decrease in the image density even when the toner density sensor is contaminated with the toner particles. This is achieved by detecting the degree of contamination and thereby adjusting the light intensity of the reflective LED (light emitting diode) light source accordingly.
U.S. Pat. No. 4,676,653, discloses a method for calibrating the light detecting measuring apparatus and eliminating errors of measurement caused by variations of the emitter or of other electronic components. This is accomplished by using one light transmitter and two detectors. A first detector measures light that is diffusely reflected off of a sample. A second detector measures light that is emitted from the light transmitter. The second detector information is used to calibrate the apparatus and to eliminate errors of measurement caused by variations in the transmitter or other electronic components.
U.S. Pat. No. 4,553,033, describes an infrared densitometer which measures the reduction in the specular component of reflectivity as toner particles are progressively deposited on a moving photoconductive belt. Collimated light rays are projected onto the toner particles. The light rays reflected are collected and directed onto a photodiode array. The photodiode array generates electrical signals proportional to the total flux and the diffuse component of the total flux of the reflected light rays. Circuitry compares the electrical signals and determines the difference therebetween to generate an electrical signal proportional to the specular component of the total flux of the reflected light rays.
Another example is U.S. Pat. No. 4,502,778, which discloses digital circuitry and microprocessor techniques to monitor the quality of toner operations in a copier and take appropriate corrective action based upon the monitoring results. Patch sensing is used. Reflectivity signals from the patch and from a clean photoconductor are analog-to-digital converted and a plurality of these signals taken over discrete time periods of a sample are stored. The stored signals are averaged for use in determining appropriate toner replenishment responses and/or machine failure indicators and controls.
U.S. Pat. No. 4,462,680, discloses a toner density control apparatus which assures always the optimum toner supply and good development with toner, irrespective of the kind of original to be copied and/or the number of copies to be continuously made. The apparatus has a detector for detecting the density of toner. The quantity of toner supply is controlled using a value variable at a changing rate different from the changing rate of the density difference between the reference toner density and the detected toner density.
U.S. Pat. No. 4,318,610, discloses an apparatus which controls toner concentration by sampling two test samples. A first test is run with a large toner concentration, wherein a second test has a smaller concentration. Developer mixture concentration is regulated in response to the first test. Photoconductive surface charging is regulated in response to the second test.
U.S. Pat. No. 4,313,671, discloses a method for controlling image density in an electrophotographic copying machine. This method uses two detectors, one measures the toner density of a blank region on a photosensitive member, the second measures a reference toner image closely positioned to the first blank region. The method then compares the two densities and uses this information to control the quantity of toner deposited thereon.
U.S. Pat. No. 4,226,541, discloses illuminating a small area of a surface to be reflectively scanned. This is followed by detecting the intensity of the light reflected from the small area and generating a first signal proportional thereto. The nest step is detecting the intensity of the light reflected from an area at least partially surrounding the small area and generating a second signal proportional thereto. Followed by subtracting at least a fraction of the second signal from the first signal to produce a compensated signal which represents the reflectivity of the small area as compensated for the effects of scattered light. Finally, the process either uses the compensated signal directly as analog data or converting it to a digital output signal having a first state when the compensated signal is above a predetermined threshold and having a second state when the compensated signal is below that threshold.
An ideal goal in electrophotography is to have the correct amount of toner deposited onto a copy sheet on a continuous basis. With poor toner development control two situations occur. First, concerning a variability of toner quantity applied, too little toner creates lighter colors, where too much color toner creates darker colors. Second, concerning the machine, too much toner development causes excess toner waste which both increases the expense of running the machine and wears parts of the machine out sooner. Machines that can achieve precise control of the toner development system will have a tremendous competitive edge.
Typically, the electrophotographic machine, or just machine, utilizes a toner monitoring system. Most commonly, as exemplified by the prior described patents, a densitometer sensor is used to measure the quantity of toner applied in order to establish some feedback and control over the toner development. These machines have been successful to some extent. However, these prior toner monitoring systems have not been responsive to both changing environmental conditions and differences between individual machines. Environmental conditions are defined as, for example, relative humidity, temperature, dirt build-up on the densitometer sensors, and electronic circuit drift. Similarly, differences between individual machines, for example, involves characteristic variability between sensors, static and dynamic variations in mounting distances or angle settings of the sensor, and variability between photoreceptors and similar image bearing members; simply put, no two machines are alike. It is obvious to one skilled in the art, that these factors are responsible for skewing the readings from feedback toner monitoring control systems, which in effect, are directly responsible for regulating the amount of toner deposited on copy sheets.
In response to these problems, a need exists for a more precise toner development monitoring system which accounts for both the changing environmental conditions and the variable characteristics between individual machine components.
As a result, the present invention provides a solution to the described problems and other problems, and also offers other advantages over the prior art.